Leptomycin B (“LMB”) is an anti-tumor, anti-microbial natural product originally isolated from Steptomyces spp., as reported in Hokanson et al., U.S. Pat. No. 4,771,070 (1988) and Nettleton et al., U.S. Pat. No. 4,792,522 (1988).

LMB is the archetype of a natural product family referred to as the leptomycin family, characterized by a 2,3-dehydro-δ-valerolactone ring at one end of the molecule (C1-C5) and an extended carbon chain having a 6E,8Z and a 12E,14E diene system located off C5. A nitromethyl valerolactone LMB analog has been found to be inactive, whereas biotinylated LMB has been found to be active, suggesting that the 2,3-dehydro-δ-valerolactone structure is a crucial pharmacophore. Kudo et al., Exp. Cell Res. 1998, 242, 540-547.

Other members of the leptomycin family include leptomycin A, ratjadone, anguinomycins A-D, callystatin A, kazusamycin A (also known as CL-1957B), kazusamycin B (also known as CL-1957E), leptolstatin, and leptofuranins A-D. The formulae of the other family members most structurally similar to leptomycin B are shown:
Leptomycin A:Ra = CH3Rb = CH3Rc = HAnguinomycin A:Ra = CH3Rb = HRc = HAnguinomycin B:Ra = CH2CH3Rb = HRc = HKasuzamycin A:Ra = CH2CH3Rb = CH3Rc = OHKasuzamycin B:Ra = CH3Rb = CH3Rc = OH
Although originally identified as a result of screening for antimicrobial activity, current interest in LMB resides primarily in its potential as an anti-tumor agent. See, e.g., Komiyama et al., J. Antibiotics 1985, 38 (3), 427-429; Wang et al., US 2003/0162740 A1 (2003). At the cellular level, LMB acts by arresting cells at the end of the G1 and G2 phases of the cell cycle. At the molecular level, LMB acts as an inhibitor of the nuclear export receptor CRM1, which binds to and affects the nuclear translocation of “cargo proteins” such as P53, P73, STAT1, (i) ADAR1, Rev, actin, and Bcr-abl. Nishi et al., J Biol. Chem. 1994, 269 (9), 6320-6324; Fukuda et al., Nature 1997, 390, 308-311; Kudo et al., cited supra.
However, LMB exhibits remarkable cytotoxicity towards mammalian cells (Hamamoto et al., J. Antibiotics 1983, 36 (6), 639-645), tempering its attractiveness as an anti-cancer agent. Thus, a phase 1 trial of LMB was halted in 1994 due to extreme toxicity. In an effort to identify more promising anti-cancer agents that exhibit LMB-like activity but are less toxic, LMB was subjected to a bioconversion screening with a number of bacteria and fungi, from which a number of derivatives were isolated (Kuhnt et al., Applied Environ. Microbiol. 1998, 64 (2), 714-720): 26-hydroxyleptomycin B, 4,11-dihydroxyleptomycin B, 2,3-dihydroleptomycin B, and leptomycin B glutaminamide.

This approach suffers from several drawbacks. The structural diversity in the products obtained was poor: the types of functional groups introduced were limited and the positions into which they were introduced were haphazard, precluding the systematic derivation of a structure-activity relationship. The number of compounds obtained in return for the effort expended was small (four compounds from a screening involving a total of 101 bacterial and fungal strains). The bioconversion yields were often low. Thus, an alternative approach to obtaining leptomycin compounds for use as an anti-cancer agent is desirable.
The prior art is generally devoid of disclosures relating to LMB esters. Kudo et al., Experimental Cell Research, 1998, 242, 540-547 (citing Schaumber et al., J. Chem. Soc. Chem. Commun., 1984, 1450-1452) refers to the methyl ester of LMB, not as a biologically active molecule, but instead as an intermediate for the synthesis of another leptomycin compound that was itself inactive.